1. Field of the Invention
The present invention relates in general to a white balance control circuit for a video camera, and more particularly to a white balance control circuit for adjusting a color temperature estimating range to compensate for errors in white balance control voltages in the video camera.
2. Description of the Prior Art
FIG. 1, shows a circuit diagram of a typical color processing circuit for a video camera. As shown in this drawing, the color processing circuit comprises a band pass filter 1 for band pass-filtering a color signal, a color amplifier 2 for amplifying an output signal from the band pass filter 2, a phase modulator 3 for modulating a phase of a synchronous signal Sp, first and second amplifiers 4 and 5 for amplifying an output signal from the phase modulator 3 respectively, first and second synchronous signal detectors 6 and 7 for detecting a color synchronous signal from an output signal from the color amplifier 2 respectively in accordance with output signals from the first and second amplifiers 4 and 5, first and second luminance signal gain controllers 8 and 9 for controlling a gain of a luminance signal Y.sub.L respectively in accordance with white balance control voltages V2 and V1, first and second matrix circuits 10 and 11 for combining output signals from the first and second synchronous signal detectors 6 and 7 with output signals from the first and second luminance signal gain controllers 8 and 9, respectively, to output difference signals B-Y.sub.L and R-Y.sub.L, and first and second color gain controllers 12 and 13 for controlling gains of output signals from the first and second matrix circuits 10 and 11 respectively in accordance with white balance control voltages V3 and V4.
FIG. 2, shows a circuit diagram of a conventional white balance control circuit. The illustrated white balance control circuit comprises a white balance control section 100 for comparing difference signals B-Y and R-Y with a reference signal Ref respectively and performing up/down-counting operations in accordance with the compared results to generate balance voltages Vo1 and Vo2, and a control voltage dividing section 101 for dividing the balance voltages Vo1 and Vo2 from the white balance control section 100 to generate the white balance control voltages V1 through V4 to the color processing circuit.
The white balance control section 100 includes first to third clamping and smoothing circuits 20 through 22 for clamping the difference signals B-Y and R-Y and the reference signal Ref respectively and smoothing the clamped difference signals B-Y and R-Y and the clamped reference signal Ref respectively, first and second comparators 23 and 24 for comparing the difference signals B-Y and R-Y outputted from the first and second clamping and smoothing circuits 20 and 21 with the reference signal Ref outputted from the third clamping and smoothing circuit 22 respectively, first and second clock generators 25 and 26 for generating up/down-clock signals respectively in response to output signals from the first and second comparators 23 and 24, first and second counters 27 and 28 for up/down-conting output signals from the first and second clock generators 25 and 26, and first and second digital/analog converters 29 and 30 for converting output signals from the first and second counters 27 and 28 into the analog voltages Vo1 and Vo2 respectively.
On the other hand, the control voltage dividing section 101 includes voltage dividers 31 and 34 for dividing the balance voltage vo1 outputted from the first digital/analog converter 29 in the white balance control section 100 by a predetermined level respectively to output the white balance control voltages V2 and V4 to the first luminance signal gain controller 8 and the second color gain controller 13 in the color processing circuit, an analog inverter 32 for inverting a level of the balance voltage Vo2 outputted from the second digital/analog converter 30, and voltage dividers 33 and 35 for dividing an output signal from the analog inverter 32 by a predetermined level respectively to output the white balance control voltages V1 and V3 to the second luminance signal gain controller 9 and the first color gain controller 12 in the color processing circuit. Herein, the reference numeral R45, not described, designates a resistor for compensating for the balance voltages Vo1 and Vo2.
With reference to FIG. 3, there is shown a detailed circuit diagram of the control voltage dividing section 101 shown in FIG. 2. As shown in this drawing, the resistor R45 in the control voltage dividing section 101 is connected between balance voltage Vo1 and Vo2 output terminals of the first and second digital/analog converters 29 and 30 in the white balance control section 100. Also, the first voltage divider 31 is provided with resistors R20 and R22, a second white balance control voltage V2 output terminal connected to the balance voltage Vo1 output terminal of the first digital/analog converter 29 in the white balance control section 100 through the resistors R20 and R22, a ground resistor R21 connected between the resistors R20 and R22, and resistors R23 and R24 connected between the resistor R22 and the second white balance control voltage V2 output terminal, each being applied with first and second source voltages Vcc1(5 V) and Vcc2 (2.7 V).
The third voltage divider 34 is provided with a resistor R25, a fourth white balance control voltage V4 output terminal connected to the balance voltage Vo1 output terminal of the first digital/analog converter 29 in the white balance control section 100 through the resistor R25 and the fourth white balance control voltage V4 output terminal, the resistor R26 being supplied with the second source vlotage Vcc2, resistors R28 and R29 for dividing the first source voltage Vcc1, and a resistor R27 connected between the resistors R28 and R29 at one of its sides and connected between the resistor R25 and the fourth white balance control voltage V4 output terminal at the other of its sides.
The analog inverter 32 is provided with a resistor R30, a transistor Q1 including its collector and base connected commonly to the balance voltage Vo2 output terminal of the second digital/analog converter 30 in the white balance control section 100 through the resistor R30 and its emitter connected to ground via a resistor R31, and a transistor Q2 including its base connected, in common with the base and the collector of the transistor Q1, to the balance voltage Vo2 output terminal of the second digital/analog converter 30 in the white balance control section 100 through the resistor R30, its emitter connected to ground via a resistor R32 and its collector for outputting a signal into which the balance voltage Vo2 from the second digital/analog converter 30 is inverted.
Also, the second voltage divider 33 is provided with resistors R33 and R34 connected between the collector of the transistor Q2 in the analog inverter 32 and the first source voltage Vcc1 terminal, a first white balance control voltage V1 output terminal connected between the resistors R33 and R34 via a resistor R35, a resistor R36 connected between the resistor R35, a resistor R36 connected between the resistor R35 and the first white balance control voltage V1 output terminal, the resistor R36 being applied with the second source voltage Vcc1, and a resistor R37 connected between the resistor R35 and the first white balance control voltage V1 output terminal at its one side and connected between the resistors R38 and R39 at its other side.
On the other hand, the fourth voltage divider 35 is provided with a resistor R40, a third white balance control voltage V3 output terminal connected to the collector of the transistor Q2 in the analog inverter 32 via the resistor R40, a resistor R41 connected between the resistor R40 and the third white balance control voltage V3 output terminal at one of its sides and connected between the resistors R43 and R44 at the other of its sides.
The operation of the conventional white balance control circuit with the above-mentioned construction will be described.
The color processing in the video camera is performed by the circuit shown in FIG. 1. As shown in FIG. 1, the color signal color is inputted by the 4.77 MHz band pass filter 1 and the filtered color signal is then amplified by the color amplifier 2. Also, the synchronous signal Sp is phase-modulated by the phase modulator 3, so that the synchronous signal Sp has a phase appropriate to the color signal processing in the video camera. The phase-modulated synchronous signal is amplified by the first and second amplifiers 4 and 5 by the corresponding gains, respectively. In accordance with the output signals from the first and second amplifiers 4 and 5, the first and second synchronous signal detectors 6 and 7 detect the synchronous signals of blue(B) and red(R) signals, respectively, from the output signal from the color amplifier 2. At this time, the luminance signal Y.sub.L is gain-controlled by the first and second luminance signal gain controllers 8 and 9, respectively, in accordance with the white balance control voltages V2 and V1, for the purpose of respective combination with the B and R signals.
Then, the output signals from the first and second synchronous signal detectors 6 and 7 and the output signals from the first and second luminance signal gain controllers 8 and 9 are combined respectively into B-Y.sub.L and R-Y.sub.L signals by the first and second matrix circuits 10 and 11. The output signals from the first and second matrix circuits 10 and 11 are gain-controlled respectively by the first and second color gain controllers 12 and 13, respectively, in accordance with the white balance control voltages V3 and V4. As a result, the white balance-controlled B-Y.sub.L and R-Y.sub.L signals are outputted from the first and second color gain controllers 12 and 13.
As mentioned above, the color processing circuit for the video camera requires the white balance control voltages V1-B4 for the purpose of the control of the white balance. The white balance control voltages V1 through V4 are conventionally supplied by the white balance control circuit shown in FIG. 2.
First in the white balance control section 100 of the white balance control circuit, the difference signals B-Y and R-Y and the reference signal Ref are clamped and smoothed by the clamping and smoothing circuits 20 through 22. Then, the clamped and smoothed difference signal B-Y and the clamped and smoothed reference signal Ref are compared with each other by the first comparator 23 and the clamped and smoothed difference signal R-Y and the clamped and smoothed reference signal Ref are compared with each other by the second comparator 24. The first comparator 23 outputs a high level signal if the level of the clamped and smoothed difference signal B-Y is over the level of the clamped and smoothed reference signal Ref; a low level signal if the level of the clamped and smoothed difference signal B-Y is below the level of the clamped and smoothed reference signal Ref. Also, the second comparator 24 outputs a low level signal if the level of the clamped and smoothed difference signal R-Y is over the level of the clamped and smoothed reference signal Ref; a high level signal if the level of the clamped and smoothed difference signal R-Y is below the level of the clamped and smoothed reference signal Ref. As a result, the first and second clock generators 25 and 26 generate up-count clock signals when the first and second comparators 23 and 24 output the high level signals; down-count clock signals when the first and second comparators 23 and 24 output the low level signals. The first and second up/down counters 27 and 28 perform up/down-counting operations as a result of the levels of output signals from the first and second clock generators 25 and 26. The counted values from the first and second up/down counters 27 and 28 are converted respectively into analog signals, Vo1 and Vo2, by the first and second digital/analog converters 29 and 30.
In other words, the balance voltage vo1 is increased in level by up-counting operation when the level of the difference signal B-Y is above the level of the reference signal Ref; is reduced in level by down-counting operation when the level of the difference signal B-Y is below the level of the reference signal Ref. On the other hand, the balance voltage Vo2 is decreased in level by down-counting operation when the level of the difference signal R-Y is above the level of the reference signal Ref; is increased in level by up-counting operation when the level of the difference signal R-Y is below the level of the reference signal Ref. The balance voltages Vo1 and Vo2 are then respectively divided into the white balance control voltages V1 through V4 by the control voltage dividing section 101, each being supplied to the color processing circuit. Namely, the balance voltage Vo1 is divided into the white balance control voltages V2 and V4 by the first and third voltage dividers 31 and 34 and the balance voltage Vo2 is inverted by the analog inverter 32 and then divided into the white balance control voltages V1 and V3 by the second and fourth voltage dividers 33 and 35.
To explain it in more detail, as shown in FIG. 3, the balance voltage Vo1 is divided into the white balance control voltage V2 by the resistors R20 through R24 and the first and second source voltages Vcc1 (5 V) and Vcc2 (2.27 V) in the first voltage divider 31 and into the white balance control voltage V4 by the resistors R25 through R29 and the first and second source voltages Vcc1 and Vcc2 in the third voltage divider 34, and then the white balance control voltages V2 and V4 are supplied respectively to the first luminance signal gain controller 8 and the second color gain controller 13 in the color processing circuit. On the other hand, the balance voltage Vo2 is inverted by the transistors Q1 and Q2 and the resistors R30 through R32 in the analog inverter 32 and then divided into the white balance control voltage V1 by the resistors R33 through R39 and the first and second source voltages Vcc1 and Vcc2 in the second voltage divider 33 and into the white balance control voltage V3 by the resistors R40 through R44 and the first and second source voltages Vcc1 and Vcc2 in the fourth voltage divider 35, and then the white balance control voltages V1 and V3 are supplied respectively to the second luminance signal gain controller 9 and the first color gain controller 12 in the color processing circuit.
However, the conventional white balance control circuit has disadvantages as follows:
First, the white balance control voltage values are different from those proposed by the designer due to natural errors of the resistors, in that the control voltage dividing section is comprised of the first and second source voltage terminals and a multiplicity of resistors in an overlapping form.
Second, the color temperature estimating range cannot be adjusted, in that the white balance control voltage ranges are fixed as the resistors are designed with fixed values. Preferably, the white balance control voltage ranges must become wider as the color temperature ranges become wider.